Serious weirdness... any insight?

So I (F 20, 5"6") came home from college a little less than a month ago, and up until the middle of this week, I was completely sedentary... I basically sat in bed or on the couch all day (I'm lazy, I know, but I was depressed :). I was also eating way above where I usually do, like from 1800-2400 calories a day. Despite these conditions, my weight remained quite stable at around 133-135 lbs.

Here's the weird part I find utterly inexplicable: This Wednesday, I finally got my act together and got a job at the library which is about 7 non-stop hours of constant, rather intense physical labor-- constant lifting heavy objects, bending, reaching, running around. I did this both on WEd. and today (Fri.). I also worked out at the gym (w/ a personal trainer--free trial) really hard for an hour and a half on Thurs. I have also been eating on the lower end of 1300-1700 cals a day during this time. For the past three days I have been at 141. (And muscles I didn't even know I had are screaming in pain--lol.)How does one gain and keep 5-8 lbs doing this in such a short time span?

Sorry this is long, but I am SO perplexed, and CCers always have such sage insight, I thought I ought to ask. :)

I always gain about 5 pounds when I get off my can and start working out and eating right.  No good eplanation unless it's just extra water ... which is what I like to tell myself.  ;-)

Is it possible that my metabolism is so slow that all of that inactivity/overeating is just now catching up with me? Or is that just silly paranoia?

Are you weighing on 2 different scales?  ALL scales will weigh you a different weight. What is important is the change over time. 

the body is a strange thing! 

A similar thing happened to me, I joined weight watchers and the gym, going to classes 3 times a week and doing 3 cardio sessions, and after 4 weeks I had put on 5 pounds. I was really annoyed.

After that the weight did start coming off though, I think that it took my body a while to get over the initial shock, and maybe it just started storing every bit of fat it could because I was suddenly eating less and doing loads of exercise, it probably went into survival mode or something!

it might just be catching up to you now, or maybe you're retaining water? 

I have a hard time figuring out which weight I actually am because mine fluctuates so much, so I use a measuring tape instead.  The measurements stay fairly consistent, but even they fluctuate a little depending on how hydrated I am.

I'm going to guess that you're retaining water.  If you're really sore, I can almost guarantee it.  You need some potassium in your diet to help you clear the lactic acid that's making you sore.  Eat some grapes, a banana, a potato...something high in potassium.  Drink lots of water.  Keep doing what you're doing and see if your scale doesn't start to budge.

your body retains more water when you're building muscle. In the long run, you'll probably see the weight coming off. The weight might not fall very much though, if you're building muscle. The upside is, you'll start looking a lot thinner even though you don't weigh much less. This is because though you're losing fats, you're also building muscles, and muscles weigh much more than fats. If it makes you feel better, you can track your progress by using a measuring tape and a scale that estimates your body fat percentage instead. Those numbers ought to be going down even if your weight doesn't. Don't worry, you're on the right track! (:

Water Water Water.  You're retaining it, you're probably a little dehydrated with all the added activity, and check your sodium intake.  Drink 2 liters of water throughout the day, and the scales will balance this out.  If you're maintaining a deficit, you're not getting fatter, you're just hoarding water because your body really thinks it's not going to get anymore for a while.  Give it what it needs and you'll be all good.

Good luck!